1. Chapter 5 유스케이스 개요 Introduction to Use Cases
박 수 용
서강대학교
컴퓨터공학과

2. Chapters in This Course
Chap 8. CMM/CMMI의 요구관리
Chap 9. 문화적 측면에서의 요구사항 관리
Chap 10. 요구관리의 해외적용사례 분석

3. What is Use Case Modeling?
A means for capturing the desired behavior for the system under development
A way to communicate the system's behavior
Identifies who or what interacts with the system and what the system should do
A way to verify all requirements are captured
A planning instrument
Actor 2
Use Case 2
Use Case 3
Use Case 1

4. Who Reads Use Cases? Client Team Developer Team Client Users Project
Manager
Technical
Writer
Tester
Requirements
Specifier
Designer

5. Benefits of Use Cases Give context for requirements
Are easy to understand
Facilitate agreement with customers
Illustrate why the system is needed
Use cases: why the system is used
Actors: who/what wants to interact with the system
The idea behind use cases is to decide what the system will be used for before defining what the system is supposed to do.

6. Actors and Use Cases Actor
Someone/something outside the system that interacts with the system
Use case
What an actor wants to use the system to do
Actor
Use Case

7. What is a Use Case? Use Case Name
A use case defines a sequence of actions performed by a system that yields an observable result of value to an actor.

8. Use Cases Contain Software Requirements
Each Use Case
Describes actions the system takes to deliver something of value to the actor
Shows the system functionality an actor uses
Models a dialog between the system and actors
Is a complete and meaningful flow of events from the perspective of a particular actor

9. Instances of Actors Sam acts as a Student Jody acts as a Student
Register for Courses

10. A User Can Act as Several Actors
acts as a Student
Student
Charlie
acts as a Professor
Professor

11. Communicates-Association
Actor 1
Use Case
Actor 2
A channel of communication between an actor and a use case
A line is used to represent it
An arrow indicates who initiates the communication

12. Each Communicates-Association is a Whole Dialog
Student logs on to system
System approves log on
Student requests course info
Course
Catalog
System
Student
Register for Courses
System displays course list
Student select courses
System confirms course availability
System displays approved schedule
System transmits request
Course Catalog returns course info

13. A Scenario is a Use Case Instance
Course
Catalog
System
Student
Register for Courses
Scenario 2
Log on to system
Approve log on
Enter subject in search
Invalid subject
Re-enter subject
Get course list
Display course list
Select courses
Confirm availability
Display final schedule
Scenario 1
Log on to system
Approve log on
Enter subject in search
Get course list
Display course list
Select courses
Confirm availability
Display final schedule

14. Use Case Diagram Apply for Financial Trading Account Network
News System
Trading
Customer
Market Trading
System
Broker
Review Account
Execute Trade
Apply for
Trading Account
Distribute
News
Manage Portfolio
Get Quote
Quote
Clock
Financial
Network

15. A Use Case Model Contains Diagrams and Text
The System
Use-Case-Model Survey
- survey description
- list of all actors
- list of all use cases
Actor 1
Actor 2
Use Case 1
Use Case 2
Actor 3
Use Case 3
Use-Case 1 Spec
- brief description
- flow of events
Use-Case 2 Spec
- brief description
- flow of events
Use-Case 3 Spec
- brief description
- flow of events

16. Example: Online Course Registration System
Student
Course
Catalog System
Register for Courses
Another Use Case
Actor X
Actor Y
Use Case 3

17. How Should I Name an Use Case?
Indicate the value or goal
Use the active form: begin with a verb
Imagine a to-do list
Examples of variations
Register for Courses
Registering for Courses
Acknowledge Registration
Course Registration
Use Registration System
Which variations show the value to the actor? Which do not?
Which would you choose as the use-case name? Why?

18. Use Case Tips Describe only the events visible to the actor:
What the actor does
What the system does in response
Make use cases provide value to an actor.
Detail until everyone has a common understanding of the requirements, then stop.
Sketch the user interface, but don’t detail it
Use agreed-upon terms and vocabulary.
Use precise language.

19. Steps for Creating a Use Case Model
1. Find actors and use cases
Identify and describe actors
Identify and describe use cases
2. Write the use cases
Outline all use cases
Prioritize and detail the use cases

20. Find Actors Who is pressing the keys (interacting with the system) ?
Student
Registrar
Registration System
The student never touches this system; the registrar operates it. Or perhaps you are building an Internet application?
Online Registration System
Student

21. Identify Actors Who/what uses the system?
Who/what gets information from this system?
Who/what provides information to the system?
Where in the company is the system used?
Who/what supports and maintains the system?
What other systems use this system?

22. Description of an Actor
Text
Name
Brief description
Relationship with use cases
Example
Student
A person who signs up for a course
Student
Register for Courses

23. What goal am I trying to achieve by using the system?
Find Use Cases
What goal am I trying to achieve by using the system?
Goal 1
Actor
Goal 2

24. Identify Use Cases What are the goals of each actor?
Why does the actor want to use the system?
Will the actor create, store, change, remove, or read data in the system? If so, why?
Will the actor need to inform the system about external events or changes?
Will the actor need to be informed about certain occurrences in the system?
Does the system supply the business with all of the correct behavior?

25. Description of a Use Case
Text description of a use case
Name
Brief description
Relationship with actors
Example
Register for Courses
The student registers for courses. The student obtains course information prior to registering.
Student
Register for Courses

26. Functional Decomposition
Is the breakdown of a problem into small isolated parts.
The parts:
Works together to provide the functionality of the system.
Often do not make sense in isolation.
Use Cases:
Are NOT functional decomposition.
Keep the functionality together to describe a complete use of the system.
Provide context.

27. Avoid Functional Decomposition
Symptoms
Very small use cases
Too many use cases
Uses cases with no result of value
Names with low-level operations
“Operation” + “object”
“Function” + “data”
Example: “Insert Card”
Difficulty understanding the overall model
Corrective Actions
Search for larger context
“Why are you building this system?”
Put yourself in user’s role
“What does the user want to achieve?”
“Whose goal does this use case satisfy?”
“What value does this use case add?”
“What is the story behind this use case?”

28. Checkpoint for Use Cases (1/2)
The use-case model clearly presents the behavior of the system; it is easy to understand what the system does by reviewing the model.
All use cases have been identified; the use cases collectively account for all required behavior.
All functional requirements are mapped to at least one use case.
The use-case model contains no superfluous behavior; all use cases can be justified by tracing  them back to a functional requirement.

29. Checkpoint for Use Cases (2/2)
Do the use cases have unique, intuitive, and explanatory names so that they cannot be mixed up at a later stage? If not, change their names.
Do customers and users alike understand the names and descriptions of the use cases?
Does the brief description give a true picture of the use case?
Is each use case involved with at least one actor?
Do any use cases have very similar behaviors or flows of events?

30. Checkpoint for Actors
Have you found all the actors? That is, have you accounted for and modeled all roles in the system's environment?
Is each actor involved with at least one use case?
Can you name at least two people who would be able to perform as a particular actor?
Do any actors play similar roles in relation to the system? If so, you should merge them into a single actor.

31. Diagram  Outline  Detail
Student
Register
for Courses
Course Catalog
System
Register for Courses Outline
-Brief description
Flow of events
Step-by-Step
Register for Courses
Use-Case Specification
-Brief description
Flow of events
Special Requirements
Pre/Post Conditions

32. Exercise 1: Identify Actors and Use Cases
Identify the actors who interact with the ATM (Automated Teller Machine)
Following features should be considered.
Withdraw, deposit, transfer, inquiry.
Identify use cases for the system
Sketch a use-case diagram
Refer to use-case and actor checkpoint slides